Method for manufacturing ferroelectric thin film device, ink jet recording head, and ink jet printer

ABSTRACT

It is an object of the present invention to control the crystal orientation of a ferroelectric thin film as dictated by the application of a ferroelectric thin film device. To accomplished the stated object, a bottom electrode containing at least iridium is formed over a surface preparation layer whose main component is zirconium oxide, and an ultra-thin titanium layer is laminated over the bottom electrode. An amorphous layer containing the elemental metal and elemental oxygen that constitute the ferroelectric is formed over the titanium layer, and a crystallized ferroelectric thin film is formed by heat treating this amorphous layer. If the thickness of the titanium layer is kept between 2 nm and 10 nm in the lamination thereof, the ferroelectric thin film will have a priority orientation of (100), and if it is kept between 10 nm and 20 nm, the ferroelectric thin film will have a priority orientation of (111).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to ferroelectric thin film device thatfunctions as a piezoelectric device, a nonvolatile ferroelectric memorydevice, a pyroelectric device, or the like, and more particularly to atechnique for controlling the orientation of a ferroelectric thin film,and to a technique for improving the bottom electrode of thisferroelectric thin film device.

2. Description of the Related Art

Crystalline materials consisting of compound oxides that exhibitferroelectricity, such as lead titanate zirconate, barium titanate, andlithium niobate, have numerous functions, including spontaneouspolarization, a high dielectric constant, an electro-optical effect, apiezoelectric effect, and a pyroelectric effect, and as such are used inthe development of a wide range of devices. For instance, thepiezoelectric properties of these materials are utilized in capacitorsin FRAM (Ferroelectric Random Access Memory), DRAM (Dynamic RandomAccess Memory), and the like, their pyroelectric properties are utilizedin infrared linear array sensors, and their electro-optical effect isutilized in wave-guide type light modulators, so these materials can beused in many different fields. Ferroelectric thin film devices havingthese various functions are also called functional devices.

It is often the case with a ferroelectric thin film device such as thisthat the characteristics vary with the crystal orientation of theferroelectric thin film. For example, most lead titanate-basedferroelectrics, which are a type of ferroelectric having a perovskitetype crystal structure, have a tetragonal crystal structure, and havespontaneous polarization in the c axis direction. Accordingly,spontaneous polarization in the direction perpendicular to the substratecan be maximized by orienting the c axis to be perpendicular to thesubstrate (c axis orientation treatment), allowing the performance of aferroelectric thin film device in which this characteristic is utilizedto be utilized to full advantage.

For this reason, it is important to control the crystal orientation inthe formation of a ferroelectric thin film, and particularly a leadtitanate-based ferroelectric film having a perovskite type crystalstructure. Furthermore, since the electrical characteristics of theseferroelectric thin films vary with the orientation of the crystal plane,an orientation treatment must be performed according to the intendedapplication of the ferroelectric thin film device. For example, a (100)priority orientation is known to be preferable with a nonvolatileferroelectric memory device that makes use of the polarizationcharacteristics of a ferroelectric thin film, such as a FRAM. With apiezoelectric device that is utilized as an electromechanical transducer(actuator), in the case of DC drive, a (111) priority orientation isknown to be preferred because of the piezoelectric constant d₃₁characteristics in the drive voltage region.

Except in the case of natural orientation, the crystal orientation of aferroelectric thin film is affected by the crystal orientation of thebottom electrode or substrate that serves as the base in the formationof the ferroelectric thin film. Accordingly, proper selection of thematerial of the bottom electrode or substrate that serves as the base isabsolutely essential to controlling the orientation of a ferroelectricthin film. Generally, a silicon substrate is used as the substrate of aferroelectric thin film, and a silicon dioxide film is formed in orderto ensure good electrical insulation between the bottom electrode andthe substrate, so a required characteristic of the bottom electrode isthat it have good orientability even when formed on an amorphous film.Platinum electrodes have been used in the past as electrodes thatsatisfy this requirement. The lattice constant of a platinum electrodeis matched to that of lead titanate zirconate, and because platinum isresistant to oxidation, no platinum oxide layer is formed at theinterface with the dielectric layer, so the performance of the devicetends not to deteriorate.

As to technology related to the bottom electrode, it has been reportedin Japanese Patent Laid-Open No. 07-245236 that a structure having aniridium layer or an alloy layer of platinum and iridium as the bottomelectrode is favorable in terms of the matching of the lattice constantsof the bottom electrode and PZT. Japanese Patent Laid-Open No. 08-335676deals with an improvement on this technology, reporting that if nucleiof a component element of PZT (titanium) are formed on the bottomelectrode in a structure having an iridium layer or an alloy layer ofplatinum and iridium as the bottom electrode, crystals will grow aroundthe nuclei and good contact with the PZT film can be ensured.

As to technology related to the substrate that serves as a base, it hasbeen reported in Japanese Patent Laid-Open No. 5-281500 that a lithiumniobate thin film is formed by sol-gel method on a sapphire (001) planemonocrystalline substrate. According to this technique, the axis ofcrystallization of a ferroelectric thin film can be uniaxially orientedby utilization of the crystallinity of the substrate.

However, even though it was possible to form a ferroelectric thin filmwith excellent orientability by optimizing the conditions that affectthe orientation of a ferroelectric thin film by means of the bottomelectrode (or substrate), such as matching the lattice constants of thebottom electrode (or substrate) and the ferroelectric thin film, as withthe above-mentioned prior art, it was difficult to control theorientation of a ferroelectric thin film as desired according to theintended application of the ferroelectric thin film. For instance, if anattempt was made to vary the film formation conditions in the formationof a PZT film by sol-gel method, it was difficult to control the (100)priority orientation, which is favorable for a nonvolatile ferroelectricmemory device, and the (111) priority orientation, which is favorablefor the DC drive of an electromechanical transducer, as desired.

Also, diligent study by the inventors revealed that when a ferroelectricthin film device is used as an electromechanical transducer, thepiezoelectric constant d₃₁ will be higher if the ferroelectric thin filmis set to a priority orientation of (111) in a drive frequency band ofjust a few kHz (low frequency band), and the piezoelectric constant d₃₁will be higher if the ferroelectric thin film is set to a priorityorientation of (100) in a drive frequency band of several dozen kHz(high frequency band). This seems to be because the piezoelectricconstant d₃₁ remains more or less constant regardless of the drivefrequency if the ferroelectric thin film is set to a priorityorientation of (100), whereas the piezoelectric constant d₃₁ decreasesin value as the drive frequency goes up if the ferroelectric thin filmis set to a priority orientation of (111). It is therefore desirable tobe able to control as desired the orientation of a ferroelectric thinfilm according to the drive frequency of the electromechanicaltransducer.

Also, the technology disclosed in Japanese Patent Laid-Open No.08-335676 allows crystals to be grown around nuclei and good contactwith a PTZ film ensured by forming nuclei of a component element of PZT(titanium) on the bottom electrode, but if iridium alone was used as thebottom electrode and the PZT film was formed by sol-gel method, thenthere was a problem in that the bottom electrode took in oxygen andswelled in the course of the baking of the PZT film. Because the bottomelectrode became hard and brittle if it took in oxygen, the bottomelectrode would break if used as an actuator.

The structure that used to be employed when a ferroelectric thin filmdevice was used as an electromechanical transducer had an adhesive layer(buffer layer) of titanium, chromium, or the like provided between thebottom electrode and the surface where this transducer was installed inorder to enhance the adhesion between the electromechanical transducerand this installation surface. The inventors of the present invention,however, discovered that in the course of the manufacture of anelectromechanical transducer, the element that makes up the adhesivelayer, such as titanium, is diffused as a result of heat treatment andmoves into the ferroelectric thin film, which diminishes thepiezoelectric characteristics of the electromechanical transducer. Thereason for this seems to be that the titanium becomes admixed with theferroelectric thin film and disrupts the stoichiometric ratio in thisfilm, or produces a layer with a low dielectric constant at theinterface between the bottom electrode and the ferroelectric thin film.

A ferroelectric thin film has spontaneous polarization, and because thepolarization direction can be inverted by the action of an externalelectrical field, this characteristic can be utilized to manufacture anonvolatile memory. When a ferroelectric thin film was applied as amemory device, an alloy of platinum and iridium, iridium alone, oriridium oxide was used in the past as a bottom electrode for applying anelectrical field to the ferroelectric thin film in an effort to enhancethe characteristics of the ferroelectric thin film and prevent theirdeterioration over time. Examples of such usage are found in U.S. Pat.No. 5,191,510, Japanese Patent Laid-Open No. 07-245287, and elsewhere.

However, the bottom electrodes composed of an alloy of platinum andiridium, iridium alone, or iridium oxide that were used for memorydevices posed problems when used as electromechanical transducers, suchas in an ink jet recording head. Specifically, the electromechanicaltransducer must itself be deformed with an ink jet recording head, butwith a bottom electrode containing iridium, there were problems in thatthe bottom electrode was too hard and the film stress generated in thebottom electrode was too high. Another problem was poor adhesion betweenthe bottom electrode and the installation surface and between the bottomelectrode and the ferroelectric thin film.

Furthermore, the problem of markedly increased leakage current wasencountered when iridium was used as the bottom electrode, as discussedin the article “Explanation of the Leakage Mechanism of a PZT CapacitorDeposited on an Ir Electrode (Lecture Summaries from the 59^(th)Convention of the Applied Physics Society, issued Sep. 15, 1998, p.450). Accordingly, the use of platinum is normally preferred for thebottom electrode when a ferroelectric thin film is used as an actuatorfor an ink jet recording head.

Platinum is therefore favorable for the bottom electrode of aelectromechanical transducer, but there has been a need for a bottomelectrode having a structure capable of preventing the admixture ofimpurities (such as titanium) into the ferroelectric thin film duringthe electromechanical transducer manufacturing process, and of enhancingthe adhesion between the bottom electrode and the installation surfaceand between the bottom electrode and the ferroelectric thin film.

When an alloy of platinum and iridium was used for the bottom electrodeof an electromechanical transducer as disclosed in Japanese PatentLaid-Open No. 07-245236, or when iridium oxide was used as disclosed inJapanese Patent Laid-open No. 07-245237, a problem was encountered inthat residual stress was generated in the bottom electrode containingiridium in the course of baking and crystallizing the ferroelectric thinfilm, and this residual stress diminished the characteristics of theelectromechanical transducer. For example, the residual stress generatedin a bottom electrode could impart strain to the ferroelectric thin filmand undesirably lower the percentage of volumetric change.

In view of this, it is an object of the present invention to provide amethod for manufacturing a ferroelectric thin film device with which thecrystal orientation of a ferroelectric thin film can be controlled asdictated by the intended application of an electromechanical transducer.It is a further object to provide an ink jet recording head whose inkdischarge drive source is an electromechanical transducer obtained bythis manufacturing method, as well as a method for manufacturing thishead, and an ink jet printer that makes use of the same. Another objectof the present invention is to provide a nonvolatile ferroelectricmemory device in which a ferroelectric thin film device obtained by thismanufacturing method serves as a capacitor, and a method formanufacturing this memory device.

Yet another object of the present invention is to provide a method formanufacturing an electromechanical transducer having very reliable drivecharacteristics when iridium alone is used as the material for thebottom electrode.

Yet another object of the present invention is to provide anelectromechanical transducer with which adhesion with the installationsurface can be enhanced without diminishing the piezoelectriccharacteristics, and an ink jet recording head and an ink jet printerthat make use of this transducer. Another object of the presentinvention is to provide a method for manufacturing an electromechanicaltransducer having a layer structure with which adhesion with theinstallation surface can be maintained without diminishing thepiezoelectric characteristics.

Still another object of the present invention is to provide anelectromechanical transducer having good piezoelectric characteristicsas a result of reduced residual stress during baking, an ink jetrecording head and printer that make use of this electromechanicaltransducer, and a method for manufacturing an electromechanicaltransducer.

SUMMARY OF THE INVENTION

With the method of the present invention for manufacturing aferroelectric thin film device, a bottom electrode film containing atleast iridium is formed on a surface preparation layer whose maincomponent is zirconium oxide, and an ultra-thin titanium layer islaminated over this bottom electrode. Next, a crystallized ferroelectricthin film is formed by forming an amorphous layer containing elementalmetal and elemental oxygen that constitute a ferroelectric over thetitanium layer, and heat treating the amorphous layer. It was confirmedthat the orientation of the ferroelectric thin film can be controlled byadjusting the film thickness during the lamination of the titanium layerat this point. For instance, if the thickness of the titanium layer isat least 2 nm and less than 10 nm, the ferroelectric thin film will havea (100) priority orientation, and if this thickness is at least 10 nmand less than 20 nm, the ferroelectric thin film will have a (111)priority orientation.

Therefore, a ferroelectric thin film device that is favorable as aelectromechanical transducer used with DC drive or low frequency drivewill be obtained by setting the thickness of the titanium layer to atleast 10 nm and less than 20 nm, whereas a ferroelectric thin filmdevice that is favorable as a capacitor for a nonvolatile ferroelectricmemory device, or an electromechanical transducer that is favorable forhigh frequency drive will be obtained by setting the thickness of thetitanium layer to at least 2 nm and less than 10 nm.

It is preferable for the ferroelectric thin film to be a ferroelectricwhose constituent components are at least titanium and lead, and leadtitanate zirconate is particularly favorable. It is preferable for theferroelectric thin film to be formed by sol-gel method. A sol-gel methodis preferred in terms of orientation control because the crystallizationof the ferroelectric thin film proceeds from the bottom electrode side.

It is also preferable for the bottom electrode to be a single layer ofan iridium film or a laminate film having a laminated structurecomprising an (iridium layer)/(platinum layer), a (platinumlayer)/(iridium layer), or an (iridium layer)/(platinum layer)/(iridiumlayer), in that order starting at the surface preparation layer.

The ink jet recording head of the present invention comprises anelectromechanical transducer obtained by the manufacturing method of thepresent invention, a pressure chamber whose internal volume is varied bythe mechanical displacement of an electromechanical transducer, anddischarge outlets that communicate with the pressure chamber and fromwhich ink droplets are discharged. The ink jet printer of the presentinvention has a printing function comprising the ink jet recording headof the present invention.

In the method of the present invention for manufacturing an ink jetrecording head, a surface preparation layer whose main component iszirconium oxide is formed on a silicon substrate surface, eitherdirectly or via a diaphragm film, and an electromechanical transducer isformed by the above-mentioned manufacturing method of the presentinvention over this surface preparation layer. The electromechanicaltransducer is then separated so as to line up with a position where themechanical displacement of the electromechanical transducer can beimparted to the pressure chamber.

In the method of the present invention for manufacturing a nonvolatileferroelectric memory device, there is a step for manufacturing thecapacitor of a memory cell by the above-mentioned manufacturing methodof the present invention.

The method of the present invention for manufacturing anelectromechanical transducer comprises the steps of forming a bottomelectrode composed of iridium alone over a surface preparation layerwhose main component is zirconium oxide, laminating a titanium layerwhose film thickness is at least 15 nm and less than 30 nm over thisbottom electrode, and forming a crystallized ferroelectric thin film byforming an amorphous film containing the elemental metal and elementaloxygen that constitute the ferroelectric over said titanium layer andthen heat treating this amorphous film.

The oxygen content of the bottom electrode in the course of baking theferroelectric thin film can be kept to a minimum and anelectromechanical transducer with excellent toughness can be provided byadjusting the thickness of the titanium layer laminated over the bottomelectrode to within a range of at least 15 nm and no more than 30 nm.

The priority orientation of the ferroelectric thin film can becontrolled to the (111) plane or the (110) plane by adjusting thethickness of the titanium layer to within the above range. The step offorming the ferroelectric thin film is preferably a sol-gel process orMOD process.

The electromechanical transducer of the present invention furthercomprises an adhesive layer formed from an alloy containing ananti-diffusion metal and formed between the bottom electrode and thesurface where the transducer is installed, and an anti-diffusion layerformed from an alloy containing the anti-diffusion metal and formedbetween the bottom electrode and said ferroelectric thin film.

The anti-diffusion metal is selected, for example, from the groupconsisting of iridium, palladium, rhodium, ruthenium, and osmium. Theabove-mentioned adhesive layer is, for example, an alloy of theanti-diffusion metal and the metal that constitutes the bottomelectrode. The above-mentioned anti-diffusion layer is, for example, analloy of the anti-diffusion metal and an adhesive metal that is eithertitanium or chronium. The bottom electrode is made of platinum.

It is preferable for the ferroelectric thin film to be formed in athickness of at least 1 μm. The baking treatment must be repeatednumerous times for this thickness to be achieved, but the diffusion ofthe titanium or other adhesive metal is prevented by the anti-diffusionlayer of the present invention even though the baking treatment isperformed numerous times.

The ink jet recording head of the present invention is constituted by anarrangement of the electromechanical transducers of the presentinvention on the diaphragm film that forms at least one side of apressure chamber filled with ink. The diaphragm film is, for example,constituted by the lamination of a silicon oxide film with a zirconiumoxide film or the like. The ink jet printer of the present invention isa printer furnished with this ink jet recording head as an ink dischargemeans.

The method of the present invention for manufacturing anelectromechanical transducer comprises the steps of forming an adhesivemetal layer composed of an adhesive metal over the surface where thetransducer is installed, forming a first anti-diffusion metal layercomposed of an anti-diffusion metal over the adhesive metal layer,forming the bottom electrode over the anti-diffusion metal layer,forming a second anti-diffusion metal layer composed of theanti-diffusion metal over the bottom electrode, and baking theferroelectric thin film while this ferroelectric thin film is formedover the second anti-diffusion metal layer, and thereby diffusing theadhesive metal all the way to the second anti-diffusion metal layer andproducing an anti-diffusion layer at the location of the secondanti-diffusion metal layer, promoting the alloying of the anti-diffusionmetal and the bottom electrode, and producing an adhesive layer at thelocation of the adhesive metal layer and first anti-diffusion metallayer.

Preferably, a metal selected from the group consisting of iridium,palladium, rhodium, ruthenium, and osmium is used as the anti-diffusionmetal. Also, either titanium or chronium is used as the adhesive metal.

The electromechanical transducer of the present invention comprises aninterlayer formed from a compound selected from the group consisting ofzirconium oxide, tantalum oxide, silicon nitride, and aluminum oxide andformed on the surface where the transducer is installed, and a bottomelectrode formed over this interlayer. The bottom electrode comprises afirst layer composed of an alloy of iridium and a specific metal andprovided over the interlayer, and a second layer containing iridium andprovided over the first layer.

This structure is formed when the baking is performed at a relativelylow temperature of 750° C. or lower, for example, and there is littlemovement of the iridium.

The electromechanical transducer in another embodiment of the presentinvention comprises an interlayer formed from a compound selected fromthe group consisting of zirconium oxide, tantalum oxide, siliconnitride, and aluminum oxide and formed on the surface where thetransducer is installed, and a bottom electrode formed over theinterlayer. The bottom electrode comprises a first layer containing aspecific metal and provided over the interlayer, and a second layercontaining iridium and provided over the first layer.

This structure is formed when the baking is performed at a relativelyhigh temperature of over 750° C., for example, and there is muchmovement of the iridium.

The above-mentioned second layer is characterized in that the iridiumthat has diffused from the lower layer side is separated from theiridium present from the outset. An adhesive layer composed of a metalthat adheres to both the interlayer and the bottom electrode may also beformed between these layers.

For example, it can be checked that the volumetric ratio in the bottomelectrode accounted for by the alloy containing iridium is at least ⅖and no more than ⅘. The “alloy containing iridium” refers to an alloy ofiridium and titanium, oxygen, or the like.

The ink jet printer of the present invention is characterized in thatthe electromechanical transducer of the present invention is provided asan actuator over the diaphragm film that serves as the installationsurface. The ink jet printer is also characterized by comprising thisink jet recording head as a printing means.

The method of the present invention for manufacturing anelectromechanical transducer comprises the steps of using a compoundselected from the group consisting of zirconium oxide, tantalum oxide,silicon nitride, and aluminum oxide to form an interlayer on the surfacewhere this transducer is installed, forming a bottom electrode over thisinterlayer, forming a ferroelectric thin film precursor over this bottomelectrode, and baking.

Here, the step of forming the bottom electrode comprises the steps ofusing iridium to form a first iridium layer, using a specific metal toform a metal layer over the first iridium layer, and using iridium toform a second iridium layer over the metal layer, the baking step beinga step of forming the ferroelectric thin film precursor and then bakingit at a temperature of 750° C. or lower, thereby diffusing the iridiumof the first iridium layer and converting the first iridium layer andthe metal layer into an alloy layer in which iridium is alloyed with themetal.

The method for manufacturing an electromechanical transducer in anotherembodiment of the present invention comprises the steps of using acompound selected from the group consisting of zirconium oxide, tantalumoxide, silicon nitride, and aluminum oxide to form an interlayer on thesurface where this transducer is installed, forming a bottom electrodeover the interlayer, forming a ferroelectric thin film precursor overthe bottom electrode, and baking.

Here, the step of forming the bottom electrode comprises the steps ofusing iridium to form a first iridium layer, using a specific metal toform a metal layer over the first iridium layer, and using iridium toform a second iridium layer over the metal layer.

The baking step is a step of forming the ferroelectric thin filmprecursor and then baking it at a temperature higher than 750° C.,thereby diffusing the iridium of the first iridium layer and moving theiridium of the first iridium layer to the second iridium layer.

Preferably, the ratio of the thickness of the first iridium layer priorto baking to the thickness of the bottom electrode overall is set to bebetween ⅓ and ⅘. The reason for this is that stress will be moderatedmore efficiently if the thickness of the iridium layer is within thisrange.

The method of the present invention for manufacturing anelectromechanical transducer may further comprise the step of using ametal that will adhere to the layers above and below to form an adhesivelayer between the bottom electrode and the interlayer.

Here, the step of forming a bottom electrode is a step of forming a filmsuch that the following relationship is satisfied:

dT=3.6×d ₀+2.4×d ₁+0.8×d ₂+2.3×d ₃

when we let d₀ be the thickness of said adhesive layer prior to baking,d₁ be the thickness of said first iridium layer, d₂ be the thickness ofsaid metal layer, d₃ be the thickness of said second iridium layer, anddT be the thickness of said bottom electrode overall after baking. Thereason for this is that the thickness of the layer after crystallizationvaries as indicated by the above relationship as a result of bakingafter formation in this relationship.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural diagram of an ink jet printer;

FIG. 2 is an exploded oblique view of an ink jet recording head;

FIGS. 3A-3F are cross sectional diagrams of the main steps formanufacturing an ink jet recording head;

FIG. 4 is a graph of the degree of (111) and (100) orientation of a PZTfilm when the titanium film thickness is varied;

FIG. 5 is a graph of the piezoelectric constant d₃₁ of a PZT film whenthe titanium film thickness is varied;

FIG. 6 is a graph of the distribution of the constituent elements in thefilm thickness direction of an electromechanical transducer;

FIG. 7 is a graph of the distribution of the constituent elements in thefilm thickness direction of an electromechanical transducer;

FIGS. 8A-8D are cross sectional diagrams of the steps for manufacturingan FRAM memory cell;

FIG. 9 is a cross sectional diagram of an electromechanical transducer;

FIGS. 10A-10F are cross sectional diagrams of the steps formanufacturing an electromechanical transducer;

FIGS. 11G-11K are cross sectional diagrams of the main steps formanufacturing an ink jet recording head;

FIG. 12 is a drawing of a cross sectional TEM photograph of an ink jetrecording head;

FIG. 13 is a compositional analysis graph for the adhesive layer afterheat treatment;

FIG. 14 is a compositional analysis graph for the bottom electrode afterheat treatment;

FIG. 15 is a compositional analysis graph for the adhesive layer afterheat treatment;

FIG. 16 is a cross sectional diagram of an electromechanical transducer;

FIGS. 17A-17F are cross sectional diagrams of the steps formanufacturing an electromechanical transducer;

FIG. 18 is a cross sectional diagram of an electromechanical transducer;

FIG. 19 is a cross sectional diagram of the step for manufacturing anelectromechanical transducer;

FIG. 20 is a measurement graph illustrating the relationship of thepiezoelectric constant and the thickness of the first iridium layer withrespect to the overall thickness of the bottom electrode prior to bakingin an embodiment of baking at 700° C.;

FIG. 21 is a measurement graph illustrating the relationship of thepiezoelectric constant and the volumetric ratio of the bottom electrodeafter baking accounted for by the alloying containing iridium in anembodiment of baking at 700° C.;

FIG. 22 is a measurement graph illustrating the relationship of thepiezoelectric constant and the thickness of the first iridium layer withrespect to the overall thickness of the bottom electrode prior to bakingin an embodiment of baking at 850° C.; and

FIG. 23 is a measurement graph illustrating the relationship of thepiezoelectric constant and the volumetric ratio of the bottom electrodeafter baking accounted for by the alloying containing iridium in anembodiment of baking at 850° C.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 of the Invention

This embodiment pertains to a technique in which a ferroelectric thinfilm device is utilized as an ink discharge drive source(electromechanical transducer) for an ink jet recording head. Thisembodiment will be described through reference to FIGS. 1 to 7.

FIG. 1 is a structural diagram of an ink jet printer. The ink jetprinter chiefly comprises an ink jet recording head 100, a main unit102, a tray 103, and a head drive mechanism 106. The ink jet recordinghead 100 is equipped with ink jet cartridges 101 of four colors: yellow,magenta, cyan, and black, which make full-color printing possible. Thisink jet printer is also internally equipped with a dedicated controllerport or the like, which controls the timing of ink discharge from theink jet recording head 100 and the scanning of the head drive mechanism106, and affords high-precision ink control, half-tone processing, andso on. The main unit 102 is equipped with the tray 103 at its rear, andwith an automatic sheet feeder (automatic continuous paper feedmechanism) 105 internally, which automatically feeds out recording paper107, with this recording paper 107 being discharged from a dischargeslot 104 at the front. The recording paper 107 can be ordinary paper,special paper, recommended OHP sheets, glossy paper, glossy film, levelsheets, standard-size postcards, or the like.

Next, FIG. 2 is an exploded oblique view of an ink jet recording head.The type shown here has a common channel for the ink provided on insideof the pressure chamber substrate. As shown in this figure, the ink jetrecording head is constituted by a pressure chamber substrate 1, anozzle plate 2, and a base 3. On the pressure chamber substrate 1, asilicon monocrystalline substrate is etched, after which are separatedfrom one another. A plurality of slender pressure chambers 10 areprovided to the pressure chamber substrate 1, and a common channel 12for supplying ink to all of the pressure chambers (cavities) 10 isprovided. Side walls 11 are erected between the pressure chambers 10 tokeep them apart. Electromechanical transducers are attached as inkdischarge drive sources to the base 3 of the pressure chamber substrate1. These electromechanical transducers are ferroelectric thin filmdevices manufactured according to the method of the present invention,and are devices for converting electrical energy into mechanical energy.These transducers are also called piezoelectric actuators ormicro-displacement control devices. The wiring from the variouselectromechanical transducers is bundled on a wiring board 4 which is aflexible cable, and is controlled by a print engine component.

The nozzle plate 2 is laid over the pressure chamber substrate 1.Nozzles (discharge ports) 21 for discharging ink droplets are formed atlocations corresponding to the pressure chambers 10 in the nozzle plate2. The nozzles 21 are arranged in rows that are more or less parallel tothe main scanning direction of the ink jet recording head duringprinting, and the pitch between the nozzles is suitably set according tothe printing precision. For instance, if the resolution is set at1440×720 dpi, the nozzle diameter will be extremely fine, andhigh-precision printing with ultra-fine ink dots will be possible. Thenumber of nozzles of each color is determined according to the colorprinting precision; for instance, there can be 32 black nozzles and 32nozzles of each color. The base 3 is molded from plastic or the like,and serves as an attachment stand for the pressure chamber substrate 1.

FIG. 3F is a cross sectional diagram of the main components of the inkjet recording head. The pressure chamber 10 is formed by etching in thepressure chamber substrate 1. The electromechanical transducer 13 isformed on top of the pressure chamber 10 via a diaphragm film 5 and asurface preparation layer 6. The mechanical displacement of theelectromechanical transducer 13 changes the internal volume of thepressure chamber 10 and discharges the ink contained in the pressurechamber 10 from the nozzle 21. The purpose of the surface preparationlayer 6 is to prepare the surface for the formation of theelectromechanical transducer 13, and plays a particularly important rolein the orientation of the ferroelectric thin film of the presentinvention. The role of the surface preparation layer 6 will be discussedin more detail below. The electromechanical transducer 13 comprises abottom electrode 71, a titanium layer 8, a ferroelectric thin film 9,and an top electrode 72. The ferroelectric thin film 9 is subjected to a(111) priority orientation treatment for the (111) plane or the (100)plane by the manufacturing method of the present invention.

A ferroelectric having at least titanium and lead as its constituentcomponents is preferable as the type of the ferroelectric thin film 9.Favorable examples include lead titanate (PbTiO₃), lead titanatezirconate (Pb(Zr,Ti)O₃), lead lanthanum titanate ((Pb,La),TiO₃), leadlanthanum titanate zirconate ((Pb,La)(Zr,Ti)O₃), and leadzirconium-titanate magnesium-niobate (Pb′(Zr,Ti)(Mg,Nb)O₃). Leadtitanate zirconate (PZT) is particularly favorable.

The bottom electrode 71 either consists of a single layer of iridiumfilm, or has a laminated structure comprising an (iridiumlayer)/(platinum layer), a (platinum layer)/(iridium layer), or an(iridium layer)/(platinum layer)/(iridium layer), in that order startingat the surface preparation layer 6. An alloy of iridium and platinum isalso acceptable. The lattice constant of the ferroelectric thin film 9can be matched by increasing or decreasing the iridium and platinumproportional contents. There are no particular restrictions on the topelectrode 72 as long as it is a conductive material that can be used asan ordinary electrode, but examples include Pt, RuO₂, IrO₂, and othersingle-layer films, or Pt/Ti, Pt/Ti/TiN, Pt/TiN/Pt, Ti/Pt/Ti,TiN/Pt/TiN, Pt/Ti/TiN/Ti, RuO₂/TiN, IrO₂/Ir, IrO₂/TiN, and otherlaminated films of two or more layers. An ultra-thin titanium thin film,a chromium thin film, or another suitable buffer layer may be interposedbetween the surface preparation layer 6 and the bottom electrode 71 inorder to increase the adhesive strength therebetween. It is alsopossible to use a structure in which the diaphragm film 5 is omitted andthe surface preparation layer 6 doubles as a diaphragm film. Also, whilenot shown in the figure, a passivation film can be used to cover theelectromechanical transducer 13 and the entire surface preparation layer6 that is exposed on the surface. A fluororesin, silicon oxide film,epoxy resin, and the like are favorable as this passivation film.

Working Example

The steps for manufacturing the electromechanical transducer in thisembodiment will now be described, along with the main steps formanufacturing an ink jet recording head, through reference to FIG. 3.First, as shown in FIG. 3A, a diaphragm film 5 and a surface preparationlayer 6 were formed on a pressure chamber substrate 1. A siliconemonocrystalline substrate with a diameter of 100 mm and a thickness of220 μm was used, for example, as the pressure chamber substrate 1. Forthe diaphragm film 5, dry oxygen was allowed to flow into a 1100° C.furnace to conduct thermal oxidation for about 22 hours, forming athermal oxidation film with a thickness of approximately 1 μm.Alternatively, a film may be formed by suitably choosing anotherformation method such as CVD. The diaphragm film 5 is not limited to asilicon dioxide film, and may instead be a zirconium oxide film, atantalum oxide film, a silicon nitride film, or an aluminum oxide film.Next, in order to form a film of zirconium oxide (ZrO₂) as the surfacepreparation layer 6, a film was formed in a thickness of about 400 nm byreactive sputtering involving the introduction of oxygen gas and usingzirconium as the target. Other film formation methods may be usedinstead, such as forming a film by RF sputtering with a zirconium oxidetarget, or forming a film of zirconium by DC sputtering and thensubjecting this film to thermal oxidation.

Next, a bottom electrode 71 was formed over the surface preparationlayer 6 (FIG. 3B). This bottom electrode 71 had a laminate structurecomprising platinum/iridium (starting from the surface preparation layer6 side), with each film being about 100 nm thick. Alternatively, thelaminate structure may comprise iridium/platinum oriridium/platinum/iridium (from the surface preparation layer 6 side), ora film may be formed by sputtering using iridium alone as the target. Inthe step of heat treating the ferroelectric thin film discussed below,part of the iridium is oxidized, forming an iridium oxide layer betweencolumnar crystals of platinum. This iridium oxide prevents the oxygenfrom escaping from the ferroelectric thin film, and thereby prevents thecharacteristics of the ferroelectric thin film from deteriorating.

A titanium layer 8 was then laminated over the bottom electrode 71 (FIG.3C). DC magnetron sputtering was utilized to form the titanium layer 8,with the thickness ranging between 2 and 10 nm. The reason for keepingthe thickness within this range will be discussed below. CVD, vapordeposition, or another such film formation process can also be used toform the titanium layer 8.

Next, a ferroelectric thin film 9 and top electrode 72 were formed overthe titanium layer 8 (FIG. 3D). In this embodiment, a PZT film wasformed by sol-gel method as the ferroelectric thin film 9. A sol-gelmethod generally involves subjecting an organometal compound such as ametal alkoxide to hydrolysis in a solution system, conductingpolycondensation to grow metal-oxygen-metal bonds, and finally bakingthis to complete the production of an inorganic oxide. An advantage of asol-gel method is that it yields a uniform film at a low substratetemperature. Furthermore, adhesion to the substrate is excellent becausethe film is formed from a solution. In particular, with the presentinvention, when the ferroelectric thin film is formed by sol-gel method,crystallization will proceed successively from the bottom electrode sidetoward the top electrode side, so control of orientation is superior. Inspecific terms, the substrate is coated with a solution containing anorganometal compound, and the coating is dried and then baked. Examplesof organometal compounds that can be used include metal methoxides,ethoxides, propoxides, butoxides, and other such alkoxides thatconstitute an inorganic oxide, and acetate compounds. Nitrates,oxalates, perchlorates, and other such inorganic salts may also be used.To produce an inorganic oxide from these compounds, it is necessary topromote hydrolysis and polycondensation reactions, and therefore watermust be added to the coating solution. The amount added will vary withthe system, but if too large, the reaction will proceed too quickly andthe resulting film will tend to be uneven, and it will also be difficultto control the reaction velocity. The reaction will also be difficult tocontrol if the amount of added water is too small, so the proper amountshould be used. The reaction velocity and reaction form can becontrolled by adding a hydrolysis acceleration catalyst, or a chelatingagent coordinated to the metal atoms. Common acids and bases can be usedas hydrolysis acceleration catalysts. The film quality will be greatlyaffected by the type of catalyst. Examples of chelating agents includeacetylacetone, ethyl acetoacetate, and diethyl malonate. The solventshould be one in which the above-mentioned materials will notprecipitate, that is, one with excellent miscibility. The solutionconcentration will vary with the coating method, but in the case of spincoating, the solution viscosity should be adjusted to between a fewcentipoise and a few dozen centipoise. Removal of organic matter andcrystallization will be promoted by baking the film coating. The bakingtemperature will vary with the material, but can be lower than that usedin the baking of ordinary metal oxide powders.

In this embodiment, a mixed solution of Pb(CH₃COO)₂·3H₂O, Zr(t—OCH₄H₉)₄,and Ti(i—OC₃H₇)₄ was readied as the starting raw material for the PZTfilm. This mixed solution was applied by spin coating at 1500 rpm in athickness of 0.1 μm, and degreasing was performed at a temperature of400° C. This step was repeated five times, which yielded a gel with afilm thickness of 0.5 μm. Finally, crystallization was effected by RTA(Rapid Thermal Annealing). This treatment was conducted for 5 minutes at650° C. The ferroelectric thin film 9 was obtained in a thickness of 0.5μm through this process. When a ferroelectric thin film device is usedas an electromechanical transducer, the ferroelectric thin film is alsocalled a piezoelectric film. Next, a film of platinum was formed by DCsputtering in a thickness of 100 nm as the top electrode 72 over theferroelectric thin film 9.

Next, the top electrode 72 was spin-coated with a resist, and thisproduct was exposed and developed at the locations where the pressurechambers were to be formed, thereby producing a pattern. Using theremaining resist as a mask, the top electrode 72, ferroelectric thinfilm 9, titanium layer 8, and bottom electrode 71 were etched, and theelectromechanical transducer 13 was separated so as to correspond to theposition where the pressure chamber was to be formed (FIG. 3E). Anetching mask was then applied over the position where the pressurechamber was to be formed, and dry etching was performed using an activegas such as a parallel plate type of reactive ion etching gas, whichetched the pressure chamber substrate 1 down to a predetermined depthand formed the pressure chamber 10 (FIG. 3E). The remaining unetchedportion became the side walls 11. The etching of the pressure chambersubstrate 1 can also be accomplished by wet etching using ahigh-concentration alkali aqueous solution, such as a 5 to 40 wt %potassium hydroxide aqueous solution.

Finally, as shown in FIG. 3F, a resin or the like was used to join thenozzle plate 2 to the pressure chamber substrate 1. The nozzles 21 canbe formed by making holes at specific locations in the nozzle plate 2 bylithography, laser working, FIB working, discharge working, or the like.In the joining of the nozzle plate 2 to the pressure chamber substrate1, the nozzles 21 were positioned so that they corresponded to thespaces of the various pressure chambers 10. The ink jet recording headwas completed by attaching the pressure chamber substrate 1 and thejoined nozzle plate 2 to the base 3.

The inventors measured the degree of orientation (%) of the (100) and(111) planes of the ferroelectric thin film 9 (PZT film) formed by theabove manufacturing process with respect to changes in the thickness ofthe titanium layer 8. The measurement results are given in FIG. 4. Thedegree of orientation of the planes here is defined as the proportionaldiffraction intensity of the (100) plane, the (110) plane, and the (111)plane when the sum of the diffraction intensities of these planes is100. As shown in the figure, the proportional orientation of the (111)plane was 15 to 45% when the thickness of the titanium layer 8 waswithin a range of 2 to 10 nm, but was 45 to 90% within a range of 10 to20 nm. Meanwhile, the proportional orientation of the (100) plane was 40to 90% when the thickness of the titanium layer 8 was within a range of2 to 10 nm, but was 5 to 40% within a range of 10 to 20 nm.

It can be seen from the above results that when a ferroelectric thinfilm device is used as an electromechanical transducer, in alow-frequency drive band, the preferred range is one in which the (111)plane is the priority orientation, that is, a range in which thethickness of the titanium layer 8 is 10 to 20 nm. On the other hand, itcan be seen that when this device is used as an electromechanicaltransducer in a high-frequency drive band, or when a ferroelectric thinfilm device is used as a capacitor for a FRAM, DRAM, or other suchmemory device, the preferred range is one in which the (100) plane isthe priority orientation, that is, a range in which the thickness of thetitanium layer 8 is 2 to 10 nm.

Applications of the present invention are not, however, limited to thosegiven above, and a suitable orientation should be chosen according tothe frequency that will be used. For example, when an electromechanicaltransducer is used as an ink jet recording head, the {100} priorityorientation probably should be about 70% and the {111} priorityorientation about 30% with a drive frequency band of about 14 kHz or 28kHz (high frequency band). Here, the degree of orientation is expressedas I(XYZ)/{I(100)+I(110)+I(111)} when we let I(XYZ) express thereflection intensity of the planar orientation (XYZ) plane of theferroelectric thin film as measured by wide-angle XRD.

The inventors further measured the change in the piezoelectric constantd₃₁ of the ferroelectric thin film 9 (PZT film) with respect to changesin the thickness of the titanium layer 8. The measurement was conductedusing DC drive, the results of which are given in FIG. 5. As shown inthis figure, when the thickness of the titanium layer 8 was between 2and 10 nm, the piezoelectric constant d₃₁ was 170 pC/N to 200 N, andwhen the thickness of the titanium layer 8 was between 10 and 20 nm, thepiezoelectric constant d₃₁ was 200 pC/N to 220 pC/N. It was confirmedfrom the above results that when an electromechanical transducer isdriven by DC, the piezoelectric characteristics will be superior whenthe thickness of the titanium layer 8 is 10 to 20 nm.

The inventors also measured by SIMS analysis the distribution of thevarious component elements in the thickness direction of theelectromechanical transducer 13 manufactured by the above manufacturingprocess. The measurement results are given in FIGS. 6 and 7. FIG. 6 isthe measurement results when the thickness of the titanium layer 8 was10 nm, and FIG. 7 is the measurement results when the thickness of thetitanium layer 8 was 20 nm. The horizontal axis denotes time, but sincethe electromechanical transducer 13 was measured in the depth directionby SIMS analysis, this axis actually denotes the various layers in thefilm thickness direction, namely, the PZT film (ferroelectric thin film9)/BE (bottom electrode 71)/ZrO₂ (surface preparation layer 6)/SiO₂(diaphragm film 5). The titanium layer 8 is at the interface between thePZT film and the bottom electrode. The vertical axis denotes the amountsof element distribution. As can be seen from these figures, thedistribution of titanium (Ti) is nearly average in the film thicknessdirection within the PZT film, with the distribution amount increasingin the vicinity of the bottom electrode. It seems that the titaniumlayer 8 laminated over the bottom electrode 71 serves as a nucleus forthe crystallization of the PZT film, promoting crystallization.

The reason is not clear why the ferroelectric thin film 9 has a (111)priority orientation if the thickness of the titanium layer 8 is 10 to20 nm. The inventors conducted experiments into this, but they have yetto discover a film with which the above-mentioned effect can be obtainedon any surface preparation layer other than a zirconium oxide film. Ittherefore seems likely that a zirconium oxide film has some kind ofeffect in terms of controlling the orientation of a ferroelectric thinfilm. Nor could they obtain the above-mentioned effect with elementsother than titanium, such as the other elements that constitute aferroelectric, including lead and zirconium. They have yet to find afilm having the same function as the titanium layer 8. On the otherhand, they confirmed that the (111) priority orientation and (100)priority orientation can be controlled by setting the thickness of thetitanium layer 8 to within the above-mentioned range, regardless of thethickness of the ferroelectric thin film 9 or that of the top electrode72.

The inventors have also confirmed that the above-mentioned effect can beobtained in an MOD (Metal Organic Decomposition) process. They wereunable, however, to obtain this effect with other film formation methodsbesides a sol-gel method, although they attempted to control theorientation of a PZT film by sputtering, laser ablation, and CVD.Whereas the crystallization of a PZT film gradually proceeds from thebottom electrode side toward the top electrode side with a sol-gel orMOD process, the crystallization of the PZT film proceeds at random withCVE, sputtering, and so on, and the resulting lack of regularity incrystallization seems to be one likely cause of the difficulty incontrolling orientation.

The orientation of a PZT film can be controlled with this embodimentwithout any special manufacturing equipment being required. Furthermore,the only difference from a conventional process for manufacturing anelectromechanical transducer is the addition of a lamination process forthe surface preparation layer and the titanium layer, so the orientationof a PZT film can be controlled by a simple means.

Embodiment 2 of the Invention

A ferroelectric thin film has an extremely large dielectric constant,ranging from a few hundred to a few thousand, and when used for theinsulating film of a capacitor, the resulting capacitor has a smallsurface area and large capacity that are favorable for circuits oflarge-scale integration. A ferroelectric thin film has spontaneouspolarization, and because the polarization direction can be inverted bythe action of an external electrical field, this characteristic can beutilized to manufacture a nonvolatile memory. An FRAM in which aferroelectric thin film such as this is used as the capacitor insulatingfilm has numerous advantages over a conventional nonvolatile memory,such as better nonvolatility, higher operating speed, lower energyconsumption, and more rewrites.

This embodiment pertains to a technique in which a ferroelectric thinfilm device is utilized as a capacitor for an FRAM memory cell. Inspecific terms, a film of zirconium oxide is formed as the surfacepreparation layer of the capacitor, and a bottom electrode, titaniumthin film, ferroelectric thin film, and top electrode are formed in thatorder over this film. As discussed above, the ferroelectric thin filmcan be set to a (100) priority orientation and a film that is favorableas a capacitor insulating film can be obtained by setting the thicknessof the titanium thin film to about 2 to 10 nm. The steps formanufacturing a FRAM memory cell will now be described through referenceto FIG. 8.

Working Example

As shown in FIG. 8A, a nitride film (Si₃N₄) was formed over the entiresurface of a silicon substrate 101, after which holes were made byphotoetching in that portion of the nitride film where the active regionwas to be formed. The exposed portion of the silicon substrate 101 wasoxidized by wet oxidation using steam, thereby forming a LOCOS (LOCalOxidation of Silicon) 102. The unnecessary nitride film was thenremoved, after which a gate oxidation film with a thickness of 150 Å wasformed by HCl oxidation. Polycrystalline silicon was deposited on thesubstrate surface by the pyrolysis of monosilane (SiH₄), and phosphorus(P) was ion-implanted at a concentration of about 10²¹/cm³. After this,this polycrystalline silicon was worked as shown in FIG. 8A byphotoetching and by dry etching using CF₄ gas or the like, therebyforming a gate electrode 103. The gate electrode 103 was then masked andarsenic (As) was ion-implanted, forming a source region 104 and a drainregion 105 through self-alignment. Further, phosphate glass wasdeposited by CVD, forming an interlayer insulating film 106.

Then, as shown in FIG. 8B, the surface preparation layer 6, bottomelectrode 71, titanium layer 8, ferroelectric thin film 9, and topelectrode 72 were formed in that order over the interlayer insulatingfilm 106. Zirconium oxide was formed by sputtering in a thickness of 200nm as the surface preparation layer 6, and a laminate structure ofplatinum/iridium in a film thickness of 100 nm each was formed as thebottom electrode 71. The titanium layer 8 was then formed in a thicknessof 2 to 10 nm by DC sputtering over the bottom electrode 71. Further, aPZT film was formed by sol-gel method in a thickness of 0.2 μm as theferroelectric thin film 9. This ferroelectric thin film 9 functions as acapacitor insulating film.

Next, as shown in FIG. 8C, the laminate structure comprising the topelectrode 72, the ferroelectric thin film 9, the titanium layer 8, thebottom electrode 71, and the surface preparation layer 6 was etched byion milling, thereby forming a capacitor 107 over the gate electrode 103via the interlayer insulating film 106. Phosphate glass was deposited byplasma CVD of tetraethyl orthosilicate (TEOS) as an interlayerinsulating film 108, and contact holes communicating with the topelectrode 72 and the drain region 105 were made, after which aluminumwiring 109 was formed by DC sputtering photoprocessing, and an aluminumetching process. The source region 104 of the FRAM memory cell obtainedby the above manufacturing process was connected to a bit line, and thegate electrode 103 was connected to a word line. The top electrode 72was connected to a drive line.

The orientation of a capacitor insulating film can be controlled withthis embodiment without any special manufacturing equipment beingrequired. Furthermore, the only difference from a conventional processfor manufacturing an FRAM memory cell is the addition of a laminationprocess for the surface preparation layer and the titanium layer, so theorientation of a capacitor insulating film can be controlled by a simplemeans.

Embodiment 3 of the Invention

This embodiment pertains to a technique for manufacturing aferroelectric thin film device in which iridium alone is used for thebottom electrode. With this embodiment, a bottom electrode composed ofiridium alone is formed over a surface preparation layer composed ofzirconium oxide. A titanium layer (titanium nucleus), which is theelement that constitutes the ferroelectric thin film (piezoelectricfilm), is then laminated over the bottom electrode, and the thickness ofthe titanium layer is adjusted to about 15 to 30 nm, which prevents theadmixture of oxygen into the bottom electrode during the baking of theferroelectric thin film in a sol-gel method.

Working Example

The steps for manufacturing the electromechanical transducer are more orless the same as the electromechanical transducer manufacturing stepsdescribed in Embodiment 1, and the following description will focus onthe differences.

First, as shown in FIG. 3A, a diaphragm film 5 composed of a silicondioxide film was formed in a thickness of 1 μm over the surface of apressure chamber substrate 1 with a thickness of 220 μm, over which asurface preparation layer 6 composed of a zirconium oxide film waslaminated in a thickness of 400 nm. Then, in order to enhance theadhesion between the surface preparation layer 6 and the bottomelectrode 71, an adhesive layer (not shown in the figure) composed oftitanium was formed in a thickness of about 10 nm over the surfacepreparation layer 6.

A bottom electrode 71 was then formed from iridium alone by sputteringin a thickness of about 200 nm (FIG. 3B), after which a titanium layer 8was formed in a thickness of 20 nm (FIG. 3C). Then, as shown in FIGS.3D, E, and F, a ferroelectric thin film 9 was formed by sol-gel method,a top electrode 72 was formed by sputtering, and the laminate structureof these thin films was then etched, which separated theelectromechanical transducers according to the pressure chambers. Thepressure chamber substrate 1 was etched to form the pressure chambers10, and the nozzle plate 2 was joined.

Table 1 shows the various characteristics of an electromechanicaltransducer obtained by the above manufacturing process (present example)and an electromechanical transducer obtained by conventionalmanufacturing process (conventional example). In both cases, thethickness of the surface preparation layer 6 is 400 nm, the thickness ofthe adhesive layer is 10 nm, and the thickness of the bottom electrode71 (iridium electrode) is 200 nm. The thickness of the ferroelectricthin film 9 of the electromechanical transducer in the present exampleis 20 nm, while the thickness of the titanium layer 8 of theelectromechanical transducer in the conventional example is 5 nm.

TABLE 1 Present example Conventional example mixture of (111) planemixture of (111) plane PZT crystal orientation and (110) plane and (110)plane Thickness of bottom 210 nm 440 nm electrode after bakingPiezoelectric constant 142 pC/N 145 pC/N Young's modulus of 300 Gpa 250Gpa bottom electrode Breakage rate during 0% 7% head drive

Based on the above results, the thickness of the bottom electrode 71more than doubled in the conventional example, whereas the thickness ofthe bottom electrode 71 only increased to 10 nm in the present example.The cause of this seems to be that oxygen is taken into the bottomelectrode 71 in the step of baking the ferroelectric thin film 9 in theconventional example, which increases the thickness of the bottomelectrode 71, whereas in the present example the oxygen content in thebottom electrode 71 is suppressed by the titanium layer 8 formed in athickness of 20 nm over the bottom electrode 71. Similar results werealso obtained when the thickness of the titanium layer 8 formed over thebottom electrode 71 was between 15 and 30 nm. In addition to the sol-gelmethod, the ferroelectric thin film 9 can also be formed by MOD process.

The bottom electrode 71 becomes hard and brittle if it contains oxygen,so the electromechanical transducer may break when used as an actuator.As can be seen from the breakage rate for the electromechanicaltransducers during head drive, which is given in the above table, therate was 7% in the conventional example, whereas it was 0% in thepresent example. It was also confirmed that the Young's modulus of thebottom electrode was higher in the present example.

Due to the effect of the titanium layer 8 laminated over the bottomelectrode 71, the ferroelectric thin film 9 was a mixture of (111) planeorientation and (110) plane orientation, which differs from the mixtureof (111) plane orientation and (100) plane orientation obtained with theconventional manufacturing method.

With the embodiment described above, the oxygen content in the bottomelectrode can be minimized during the baking of the ferroelectric thinfilm by sol-gel method or MOD process by laminating a titanium layer ina thickness of 15 to 30 nm over a bottom electrode composed of iridiumalone, which means that the bottom electrode will exhibit bettertoughness, and an actuator with superior reliability can be realized.

Embodiment 4 of the Invention

This embodiment relates to the bottom electrode of an electromechanicaltransducer with which adhesion to the installation surface can beenhanced without compromising the piezoelectric characteristics.

FIG. 9 is a cross sectional diagram illustrating the layer structurewhen the electromechanical transducer pertaining to this embodiment isformed over an installation surface. As shown in the figure, anelectromechanical transducer 40 is produced by the successive laminationof an adhesive layer 41, a bottom electrode 42, an anti-diffusion layer43, a ferroelectric thin film (piezoelectric film) 44, and a topelectrode 45 over the installation surface. There are no particularrestrictions on the installation surface, but in this embodiment itcorresponds to the diaphragm of an ink jet recording head (for instance,a silicon oxide film or a laminate structure of a silicon oxide filmlayer and a zirconium oxide film).

The adhesive layer 41 is manufactured by the method of the presentinvention for manufacturing an electromechanical transducer, and isformed from an alloy containing an anti-diffusion metal. Thisanti-diffusion metal is selected, for example, from the group consistingof iridium (Ir), palladium (Pd), rhodium (Rh), ruthenium (Ru), andosmium (Os). A characteristic of these metals is that they prevent themovement of metals such as titanium and chromium, which are readilydiffused by heat, along the grain boundaries of the crystal grains ofthe above metals. The main component of the adhesive layer 41 is analloy of the above-mentioned anti-diffusion metal and the metal thatconstitutes the bottom electrode. Because platinum is used for thebottom electrode in this embodiment, the adhesive layer is an alloy ofplatinum and the above-mentioned anti-diffusion metal. However, a smallamount of the adhesive metal formed at the outset, such as titanium orchromium, remains at the location of this adhesive layer. Specifically,this adhesive layer 41 is not a layer whose composition stays constantfrom when it is first formed, and is instead a layer produced as aresult of the movement of elements occurring in the course of the heattreatment step. The thickness of the adhesive layer 41 is equal to thecombined thickness of the adhesive metal layer applied prior to the heattreatment and the first anti-diffusion metal layer.

The bottom electrode 42 is one of the electrodes used to apply voltageto the ferroelectric thin film 44, and is formed from a conductivematerial, such as platinum (Pt), in a thickness of about 0.4 μm. With aconventional product, the thickness had to be over 0.5 μm because of themovement of the adhesive metal through this bottom electrode, but withthis embodiment the anti-diffusion metal plays this role, so the filmcan be 0.2 μm thinner than in the past. The top electrode 45 is theother electrode for applying voltage to the ferroelectric thin film 44,and is formed from a conductive material, such as platinum (Pt), in athickness of about 0.1 μm.

The anti-diffusion layer 43 is formed from an alloy of an anti-diffusionmetal and an adhesive metal. The anti-diffusion metal is the same asabove, and is selected, for example, from the group consisting ofiridium, palladium, rhodium, ruthenium, and osmium. The adhesive metalis formed as an adhesive metal layer prior to heat treatment, and iseither titanium or chromium, for example. The anti-diffusion layer 43 isa layer in which a second anti-diffusion metal layer formed from ananti-diffusion metal prior to heat treatment is alloyed with theadhesive metal that has come through from the adhesive metal layer, andthereby forms crystals. The function of the second anti-diffusion metallayer, which is the forerunner to this anti-diffusion layer 43, is toprevent the adhesive metal from diffusing into the ferroelectric thinfilm 44 during the heat treatment step, and to prevent the lead oroxygen present in the ferroelectric thin film 44 from escaping into thebottom electrode 42 during the heat treatment step. The anti-diffusionlayer 43 is thus formed by the movement of elements, so the thicknessthereof varies according to the amounts in which the secondanti-diffusion metal layer and the adhesive metal layer are formed priorto the heat treatment. For instance, there will be minute fluctuationsdepending on the size of the crystal grains composed of an alloy ofanti-diffusion metal and adhesive metal.

The ferroelectric thin film 44 consists of crystals of an ordinarypiezoelectric ceramic. Favorable examples include a ferroelectricpiezoelectric material such as lead titanate-zirconate (PZT), or theproduct of adding to this a metal oxide such as magnesium oxide ornickel oxide, or niobic acid. The composition of the ferroelectric thinfilm 44 is suitably selected after taking into account thecharacteristics an and intended application of the electromechanicaltransducer. In specific terms, it is possible to use lead titanate(PbTiO₃), lead titanate zirconate (Pb(Zr,Ti)O₃), lead zirconate(PbZrO₃), lead lanthanum titanate ((Pb,La),TiO₃), lead lanthanumtitanate zirconate ((Pb,La) (Zr,Ti)O₃), lead zirconium-titanatemagnesium-niobate (Pb′(Zr,Ti)(Mg,Nb)O₃), or the like. A film withsuperior piezoelectric characteristics can be obtained by suitableaddition of niobium (Nb) to lead titanate or lead zirconate.

The ferroelectric thin film should be thin enough that cracks do notform during the manufacturing process, but should be thick enough thatit will exhibit sufficient displacement characteristics. In particular,because this embodiment involves a structure with which the adhesivemetal will not diffuse into the ferroelectric thin film even after aheat treatment has been conducted numerous times, it is possible for thefilm to be made thicker than conventional products by numerous heattreatments. In the past, adhesion was diminished by diffusion of theadhesive metal if the baking was carried out more than twice, but withthis embodiment, there is no loss of adhesion even after baking four ormore times. Accordingly, the thickness of the ferroelectric thin film 44can be 1 μm or more, for example.

Working Example

Next, a method for manufacturing an ink jet recording head, includingthe method of the present invention for manufacturing anelectromechanical transducer, will be described through reference toFIGS. 10 and 11. First, a diaphragm film 5 was formed on the surface ofa pressure chamber substrate 1 composed of a silicon monocrystallinesubstrate (FIG. 10A). In this step, a high-temperature treatment shouldbe carried out in an oxidative atmosphere containing oxygen or steam bymeans of a commonly used thermal oxidation method or the like to form athermal oxidation film composed of silicon dioxide. The result of thisstep was the formation of the diaphragm film 5 in a suitable thickness(1.0 μm, for instance) over the pressure chamber substrate 1 of aspecific thickness (220 μm, for instance). Instead of a silicon oxidefilm, it is also possible to form a zirconium oxide film, tantalum oxidefilm, silicon nitride film, or aluminum oxide film. Th e diaphragm mayalso consist of a laminate structure of a silicon oxide film and azirconium oxide film or the like. To produce a laminate structure, forexample, a silicon oxide film is formed as above in a thickness of about1 μm over a silicon monocrystalline substrate, after which a zirconiumlayer is formed by sputtering, vacuum vapor deposition, or another suchmethod. A high-temperature treatment should then be performed in anoxygen atmosphere, and a zirconium oxide film formed in a thickness ofabout 400 nm.

Next, a layer 50 was formed from an adhesive metal over the diaphragmfilm 5 (FIG. 10B). The adhesive metal here refers to titanium orchromium. These metals are readily diffused by heat treatment during thesubsequent formation of the ferroelectric thin film 44, but afterdiffusion they move to the anti-diffusion layer 43 and eventually remainin the adhesive layer 41, where they serve to bond the bottom electrode42 and the diaphragm film 5 together. Sputtering, vacuum vapordeposition, or another such method may be used to form the adhesivemetal layer 50. The adhesive metal layer 50 should be thick enough toallow an adequate supply of metal for adhesion, but not so thick thatthere will be no diffusion to the ferroelectric thin film. A thicknessof about 20 nm is good.

Next, a layer 51 of anti-diffusion metal was formed over the adhesivemetal layer 50 (FIG. 10C). This layer leaves behind part of the adhesivemetal diffused by the subsequent heat treatment, and functions as theadhesive layer 41 that enhances the adhesive strength between the bottomelectrode 42 and the diaphragm film 5. Sputtering, vacuum vapordeposition, or another such method is used to form the firstanti-diffusion metal layer 51. The first anti-diffusion metal layer 51should be thick enough that it will cause part of the adhesive metaldiffused in the direction of the ferroelectric thin film to remainbehind. A thickness of about 20 nm is good, for example.

Next, a layer 42 of a bottom electrode was formed over the firstanti-diffusion metal layer 51 (FIG. 10D). Sputtering or another suchmethod was used to form the bottom electrode 42. With a conventionalproduct, the film thickness had to be at least 500 nm in order toprevent the diffusion of titanium, but with this embodiment theanti-diffusion metal prevents the diffusion of titanium, so the film canbe formed in a thickness of 400 nm or less.

Next, a layer 52 of anti-diffusion metal was formed over the bottomelectrode 42 (FIG. 10E). This layer functions as a barrier that haltsthe diffusion of the adhesive metal that has diffused through the bottomelectrode 42 as a result of the subsequent heat treatment. Sputtering,vacuum vapor deposition, or another such method may be used to form thesecond anti-diffusion metal layer 52. The second anti-diffusion metallayer 52 should be thick enough to halt the diffusion of the adhesivemetal into the ferroelectric thin film, and a film thickness of about 20nm is good, for example. It is preferable for the adhesive metal layer,the first anti-diffusion metal layer, the bottom electrode, and thesecond anti-diffusion metal layer to be formed continuously in a vacuumin order to prevent contamination.

Next, the ferroelectric thin film 44 was formed by lamination of aferroelectric thin film by sol-gel method (FIG. 10F to FIG. 11H). Theadhesive metal layer 50 was diffused all the way to the secondanti-diffusion metal layer 52, and the anti-diffusion layer 43 producedat the location of this second anti-diffusion metal layer 52, by thebaking treatment performed for the crystallization of the ferroelectricthin film. This step also involves promoting the alloying of the firstanti-diffusion metal layer 51 with the bottom electrode 42, and therebyproducing the adhesive layer 41 at the location of the adhesive metallayer 50 and the first anti-diffusion metal layer 51.

First, a sol composed of an organometal alkoxide was applied by spincoating or another coating method over the bottom electrode 42. Thiscoating was then dried for a specific length of time at a specifictemperature to evaporate the solvent. After drying, degreasing wasperformed for a specific time at a specific high temperature in an airatmosphere, and the organic ligands coordinated to the metal werepyrolyzed, which yielded a metal oxide. These steps of coating, drying,and degreasing were repeated a specific number of times (such as four ormore) to laminate four or more layers of ferroelectric thin filmprecursor 441 to 44 n (n is the number of coatings) (FIG. 10F→FIG.11G→FIG. 10F→FIG. 11G . . . ). As a result of this drying anddegreasing, the metal alkoxide and acetate in the solution formed anetwork of metal-oxygen-metal via the pyrolysis of ligands. After theferroelectric thin film precursors had been laminated a specific numberof times, a baking treatment was performed in a specific atmosphere.This baking treatment formed a perovskite crystal structure havinganything from an amorphous gel structure to any kind of crystalstructure.

In the course of the above-mentioned heat treatment, the titanium in theadhesive metal layer 50 diffuses along the grain boundaries of thecrystals. Part of this diffusion is prevented by the firstanti-diffusion metal layer 51. Meanwhile, the platinum of the bottomelectrode 42 diffuses somewhat toward this first anti-diffusion metallayer 51. As a result, an alloy layer consisting of the diffused metal,the metal that makes up the bottom electrode, and the adhesive metal isformed at the location of the first anti-diffusion metal layer 51 andthe adhesive metal layer 50. This is the adhesive layer 41 (FIG. 11H).

The titanium in the adhesive metal layer 50 passes through the bottomelectrode 42 and reaches the second anti-diffusion metal layer 52, wherethis diffusion is stopped. Accordingly, an alloy layer of theanti-diffusion metal and the adhesive metal is formed at the location ofthe second anti-diffusion metal layer 52. This is the anti-diffusionlayer 43 (FIG. 11H).

Next, the top electrode 45 was formed by electron beam vapor deposition,sputtering, or another such method over the ferroelectric thin film 44(FIG. 11I). Platinum, iridium, or the like was used for the material ofthe top electrode, and the thickness was about 100 nm.

The basic form of the electromechanical transducer is complete at theabove step. This electromechanical transducer can be made to operate asthe piezoelectric device of the present invention if it is etched into ashape suited to the place where it is to be used and if it ismanufactured such that voltage can be applied between the top and bottomelectrodes. With this embodiment, the laminate structure of theabove-mentioned piezoelectric device was etched to match the ink jetrecording head, and the ink jet recording head was then manufactured bythe steps described below. FIG. 11 illustrates the cross sectionalstructure of three electromechanical transducers in particular, but inactual practice the number and spacing of the electromechanicaltransducers may be varied according to the resolution of the ink jetrecording head.

First, after the formation of the laminate structure of the variouselectromechanical transducers, the top electrode 45, the ferroelectricthin film 44, the anti-diffusion layer 43, the bottom electrode 42, andthe adhesive layer 41 were masked in a shape matching the pressurechambers 10, and etching was performed around this mask (FIG. 11J). Inspecific terms, first a resist material was applied over the topelectrode in a uniform thickness by spinner method, spraying, or anothersuch method, a mask was formed in the shape of the electromechanicaltransducers, and then exposure and developing were performed to form aresist pattern over the top electrode 45. This product was subjected toordinary ion milling, dry etching, or another such process to etch awaythe top electrode 45, ferroelectric thin film 44, anti-diffusion layer43, bottom electrode 42, and adhesive layer 41 and shape theelectromechanical transducers 40.

Next, the pressure chambers 10 were formed by etching the other side ofthe pressure chamber substrate 1 from that on which theelectromechanical transducers 40 were formed (FIG. 11J). For example,the spaces for the pressure chambers 10 were etched into the siliconsubstrate by anisotropic etching, parallel plate type of reactive ionetching, or another form of anisotropic etching using an active gas. Theunetched portion that remains behind becomes the side walls 11.

Finally, the nozzle plate 2 is bonded to the etched pressure chambersubstrate 1 with an adhesive agent to manufacture the main components ofan ink jet recording head (FIG. 11K). When the parts are bondedtogether, the nozzles 21 are aligned so that they will be arranged inthe spaces of the various pressure chambers 10. The pressure chambersubstrate 1 with its bonded nozzle plate 2 is attached to a housing tocomplete an ink jet recording head 1. Instead of the nozzle plate 2being bonded, the nozzle plate 2 and the pressure chamber substrate 1may be formed by integral etching. There is no need for the bonding stepwhen the nozzle plate and the pressure chamber substrate aremanufactured at the same time by integral etching. The nozzles are madeat positions corresponding to the pressure chambers 10.

In the above manufacturing method, the electromechanical transducerswere formed using titanium as the adhesive metal, platinum as the bottomelectrode, iridium as the anti-diffusion metal, and PZT-PMN as theferroelectric thin film. FIG. 12 is a drawing of a cross sectional TEM(Transmission Electron Microscopy) photograph of the crystal structurethereof. As this figure confirms, an adhesive layer has been formed inwhich alloying has proceeded between the diaphragm film and the bottomelectrode. It also confirms the formation of an anti-diffusion layerwhich is an alloy layer of anti-diffusion metal crystals and adhesivemetal at the interface between the bottom electrode and theferroelectric thin film.

FIG. 13 is a compositional analysis graph for the adhesive layer. Thisfigure relates to the composition of that region of the adhesive layerrelatively near the bottom electrode. As can be seen from the figure,with this embodiment only a tiny amount of titanium (arrow 1), which isthe composition of the adhesive metal layer that had been formed at thislocation prior to heat treatment, remains. Instead, platinum (Pt; arrow2), which is the metal that makes up the bottom electrode, and iridium(Ir; arrow 3), which is the anti-diffusion metal that had made up theanti-diffusion metal layer prior to heat treatment, are present. It canbe confirmed that the two metals have been alloyed. The heat treatmentcauses the titanium to diffuse, and the platinum in the bottom electrodeand the iridium in the anti-diffusion metal layer move in and form anadhesive layer.

FIG. 14 is a compositional analysis graph for the bottom electrode. Ascan be seen from the figure, the platinum (Pt; arrow 1) present from theoutset accounts for virtually all of the composition. Part of thetitanium (Ti; arrow 2) that is the adhesive metal that has diffused fromthe adhesive metal layer remains.

FIG. 15 is a compositional analysis graph for the adhesive layer. Asshown in this figure, the iridium (Ir; arrow 1) that is theanti-diffusion metal that had been formed in the anti-diffusion metallayer prior to heat treatment is present. Also beginning to appear isthe titanium (Ti; arrow 2) that is the adhesive metal that has diffusedfrom the adhesive metal layer. Also present is the oxygen (O; arrow 3)that has moved from the ferroelectric thin film. The composition here isa mixture of almost completely unoxidized iridium and titanium oxide.Specifically, it can be seen that the anti-diffusion layer prevents thetitanium from moving to the ferroelectric thin film, and prevents theoxygen from escaping from the ferroelectric thin film to the bottomelectrode.

Table 2 shows the change in adhesion when the composition of theanti-diffusion layer and adhesive layer was varied. In the comparativeexamples, there is no layer corresponding to the anti-diffusion layer,and the adhesive layer consists of titanium.

TABLE 2 Bottom Ferroelectric Number Adhesive Electrode electrode thinfilm of strength constitution thickness thickness bakings (mN) Compar.Example 500 nm 0.8 μm 2 17.8 Working Example 500 nm 0.8 μm 2 18.0Compar. Example 500 nm 1.2 μm 4 14.2 Working Example 500 nm 1.2 μm 418.8 Compar. Example 300 nm 1.2 μm 4 9.8 Working Example 300 nm 1.2 μm 419.8

As can be seen from Table 2, adhesion was better in the working examplesthan the comparative examples. In particular, there was a relativeincrease in adhesion when the number of bakings was increased and thethickness of the ferroelectric thin film was over 1 μm. Therefore, thelayer structure of this embodiment is also effective when theferroelectric thin film thickness is increased to over 1 μm.

Next, Table 3 shows the breakdown voltage and piezoelectric constant d₃₁of an electromechanical transducer in working examples and comparativeexamples of the above-mentioned layer structure when the thickness ofthe bottom electrode was 500 nm and the thickness of the ferroelectricthin film was varied.

TABLE 3 Ferro- electric thin Piezoelectric Piezoelectric Electrode filmBreakdown constant constant constitution thickness voltage at 15 V(pC/V) at 25 V (pC/V) Compar. 0.8 μm 41 V 178 161 Example Working 0.8 μm60 V 196 175 Example Compar. 1.2 μm 62 V 191 170 Example Working 1.2 μm93 V 240 195 Example

As can be seen from Table 3, the breakdown voltage and piezoelectricconstant d₃₁ are higher in the working examples than the comparativeexamples. The reason for this is that no low-dielectric constant layeris present at the interface between the bottom electrode and theferroelectric thin film in the electromechanical transducers of theworking examples. The reason the value of the piezoelectric constant d₃₁varies with the measured voltage is that, because the applied electricalfield intensity is high with an electromechanical transducer, there is atendency for the piezoelectric constant d₃₁ to decrease as theelectrical field increases in intensity. Thus, increasing the thicknessof the ferroelectric thin film is an effective way to enhance thecharacteristics of an electromechanical transducer. Making theferroelectric thin film too thick, however, is undesirable because thedisplacement efficiency will decrease. For example, a ferroelectric thinfilm thickness that is favorable for a high-resolution ink jet recordinghead is probably about 1 or 2 μm.

Embodiment 5 of the Invention

This embodiment relates to an electromechanical transducer formed at arelatively low temperature, to a method for manufacturing thiselectromechanical transducer, and to an ink jet recording head and inkjet printer that make use of the same.

FIG. 16 is a cross sectional diagram of a layer structure, in which theelectromechanical transducer portion of an ink jet recording head inthis embodiment has been enlarged.

As shown in this figure, an electromechanical transducer 80 comprises aninterlayer 92, an adhesive layer 81, a bottom electrode 82, aferroelectric thin film (piezoelectric film) 83, and a top electrode 84,all laminated over an oxide film 91 that forms the installation surface.

The interlayer 92 is an elastic layer, and together with the oxide film91, constitutes a diaphragm film 70. The interlayer 92 is formed from acompound selected from the group consisting of zirconium oxide, tantalumoxide, silicon nitride, and aluminum oxide. Here, it is formed fromzirconium oxide. The interlayer 92 is thick enough to function as anelastic layer (for example, a thickness between 200 and 800 nm).

The adhesive layer 81 is formed between the interlayer 92 and the bottomelectrode 82, from a metal that adheres to these two layers. Thisadhesive layer serves to improve the adhesion of the electromechanicaltransducer to the installation surface, but it need not be formed whengood adhesion can be ensured. Titanium and chromium are examples ofmetals with good adhesion. The adhesive layer 81 should be thick enoughto ensure the minimum acceptable adhesion, such as 10 nm or more.

The bottom electrode 82 consists of a first layer 824 and a second layer825. The first layer 824 and second layer 825 are formed as a result ofthe movement of the composition resulting from the manufacturing methodof this embodiment, as discussed below.

The first layer 824 is a layer composed of an alloy of iridium and aspecific metal, provided over the interlayer 92. An example of thismetal is platinum. This alloy is produced when the iridium (821)provided under the metal layer (822) of platinum or the like is moved byheat to the metal layer and alloyed there. Accordingly, the first layer824 is not uniformly mixed throughout the entire layer, and instead hasa somewhat heterogeneous crystal structure in which iridium crystals areadmixed in the crystallization of the platinum or other metal.

The second layer 825 is provided over the first layer 824, and is formedas a layer containing iridium. This layer is produced when the iridiumlayer (823) provided from the outset is crystallized as it is.

In the bottom electrode 82 consisting of the first layer 824 and thesecond layer 825, the volumetric ratio of the bottom electrode 82accounted for by the alloy containing iridium (such as an alloy of thetitanium or other metal that has moved from the adhesive layer 81, theoxygen that has escaped from the ferroelectric thin film 83, andiridium) is at least ⅖ and no more than ⅘.

The ferroelectric thin film 83 is a ferroelectric consisting of anordinary piezoelectric ceramic. Favorable examples include aferroelectric piezoelectric material such as lead titanate-zirconate(PZT), or the product of adding to this a metal oxide such as magnesiumoxide or nickel oxide, or niobic acid. The composition of theferroelectric thin film 83 is suitably selected after taking intoaccount the characteristics and intended application of theelectromechanical transducer. In specific terms, it is possible to uselead titanate (PbTiO₃), lead titanate zirconate (Pb(Zr,Ti)O₃), leadzirconate (PbZrO₃), lead lanthanum titanate ((Pb,La),TiO₃), leadlanthanum titanate zirconate ((Pb,La)(Zr,Ti)O₃), lead zirconium-titanatemagnesium-niobate (Pb′(Zr,Ti)(Mg,Nb)O₃), or the like. A film withsuperior piezoelectric characteristics can be obtained by suitableaddition of niobium (Nb) to lead titanate or lead zirconate.

The ferroelectric thin film should be thin enough that cracks do notform during the manufacturing process, but should be thick enough thatit will exhibit sufficient displacement characteristics.

The top electrode 84 is an electrode paired with the bottom electrode82, and is formed from a specific metal such as platinum or iridium. Thethickness thereof is about 100 nm.

Working Example

An example of manufacturing the electromechanical transducer and ink jetrecording head of the present invention will now be described throughreference to FIGS. 17 and 18.

First, the oxide film 91 was formed on the surface of the pressurechamber substrate 1 (FIG. 17A). In this step, a high-temperaturetreatment is carried out in an oxidative atmosphere containing oxygen orsteam to form the oxide film 91 composed of silicon dioxide. Any thermaloxidation method or CVD that is ordinarily used can be used in this stepas well. The result of this step was the formation of the oxide film 91in a suitable thickness (1.0 μm, for instance) over the pressure chambersubstrate 1 composed of a silicon monocrystalline substrate of aspecific thickness (220 μm, for instance).

Next, the interlayer 92 was formed over the oxide film 91 (FIG. 17B). Acompound selected from the group consisting of zirconium oxide, tantalumoxide, silicon nitride, and aluminum oxide was used as the material forthis interlayer. If zirconium oxide is used, for example, first azirconium layer is formed over the oxide film 91 by sputtering, vacuumvapor deposition, or another such method, and this product is treated athigh temperature in an oxygen atmosphere to produce a interlayer 92 ofzirconium oxide. The thickness thereof should be about 400 nm.

Next, the bottom electrode 82 was formed over the diaphragm film 70formed in the above step (FIG. 17C). In this example, the adhesive layer81 was further formed from an adhesive metal between theelectromechanical transducer 80 and the diaphragm film 70 in order toprevent these from separating. “Adhesive metal” refers to titanium orchromium. Sputtering, vacuum vapor deposition, or the like may be usedto form the film. The adhesive layer 81 should be thick enough for it tofunction sufficiently as an adhesive metal layer, but not so thick thatthere will be no diffusion into the piezoelectric film. A thickness ofabout 10 nm or more is good.

The step of forming the bottom electrode comprises the steps of forminga first iridium layer 821, forming a metal layer 822, and forming asecond iridium layer 823.

The first iridium layer 821 is formed by causing iridium to adhere bysputtering or another such method. The thickness of the first iridiumlayer 821 is important. Specifically, the thickness of the first iridiumlayer 821 is set such that the ratio of this thickness to the thicknessof the bottom electrode 82 as a whole will be from ⅓ to ⅘. Since notonly the thickness of this layer 821, but the thickness of the metallayer 822 and the second iridium layer 823 is also affected, thethickness of each layer is set after the required thickness of thebottom electrode 82 as a whole has been determined. Here, the thicknessis adjusted such that the following relationship is satisfied:

dT=3.6×d ₀+2.4×d ₁+0.8×d ₂+2.3×d ₃  (1)

when we let d₀ be the thickness of the adhesive layer prior to baking,d₁ be the thickness of the first iridium layer, d₂ be the thickness ofthe metal layer, d₃ be the thickness of the second iridium layer, and dTbe the thickness of the bottom electrode overall after baking. This isbecause the stress moderation that is an object of the present inventioncan be accomplished more effectively when the lamination is in theseproportions.

The metal layer 822 is formed by causing a specific conductive metal,such as platinum, to adhere by sputtering or another such method. Thereare no particular restrictions on the thickness thereof, but about 100nm is good, for example.

The second iridium layer 823 is formed by again causing iridium toadhere over the metal layer 822 by sputtering or another such method.The second iridium layer 823 should be thick enough to prevent theiridium, titanium, or other such metal that has diffused through themetal layer 822 from diffusing into the piezoelectric film. 20 nm ormore is good, for example.

It is preferable for the adhesive layer 81, the first iridium layer 821,the metal layer 822, and the second iridium layer 823 to be formedcontinuously in a vacuum in order to prevent contamination.

Next, the ferroelectric thin film 83 was formed by lamination of aferroelectric thin film by sol-gel method (FIGS. 17D and E). First, asol composed of an organometal alkoxide was applied by spin coating oranother coating method over the bottom electrode 82. This coating wasthen dried for a specific length of time at a specific temperature toevaporate the solvent. After drying, degreasing was performed for aspecific time at a specific high temperature in an air atmosphere, andthe organic ligands coordinated to the metal were pyrolyzed, whichyielded a metal oxide. These steps of coating, drying, and degreasingwere repeated a specific number of times (such as four or more) tolaminate four or more layers of ferroelectric thin film precursor. As aresult of this drying and degreasing, the metal alkoxide and acetate inthe solution formed a network of metal-oxygen-metal via the pyrolysis ofligands (FIG. 17D).

The ferroelectric thin film precursors were then baked and crystallizedat a temperature of 750° C. or lower (FIG. 17 E). This heat treatmentdiffused the iridium of the first iridium layer 821, and converted thefirst iridium layer 821 and the metal layer 822 into a first layer 824that was an alloy layer in which iridium and platinum were alloyed.

This heat treatment also resulted in the ferroelectric thin filmprecursors forming a perovskite crystal structure from their amorphousform they had when they were precursors, and changed them into a thinfilm that exhibited an electromechanical transducing action.

Finally, the top electrode 84 was formed over the ferroelectric thinfilm 83 by electron beam vapor deposition, sputtering, or another suchmethod (FIG. 17F). Platinum, iridium, or the like may be used for thematerial of the top electrode, and the thickness is about 100 nm.

The basic form of the electromechanical transducer is complete at theabove step. This electromechanical transducer can be made to operate asthe piezoelectric device of the present invention if it is etched into ashape suited to the place where it is to be used and if it ismanufactured such that voltage can be applied between the top and bottomelectrodes. Thereafter, an ink jet recording head is completed byseparating the electromechanical transducer by etching into transducerscorresponding to the pressure chambers according to the stepsillustrated in FIGS. 11J to L, and working the pressure chambersubstrate.

With this embodiment, an electrode is sandwiched by an iridium layerover an interlayer, and baked at a temperature of 750° C. or lower,which effectively moderates the residual stress in the bottom electrodethrough the diffusion of the iridium, and makes it possible to providean electromechanical transducer having good piezoelectriccharacteristic, and products to which this electromechanical transduceris applied.

Embodiment 6 of the Invention

This embodiment relates to the structure of an electromechanicaltransducer manufactured when baking is performed at a temperature over750° C. in the manufacturing process of Embodiment 5.

FIG. 18 is a cross sectional diagram of the structure of a layerstructure, in which the electromechanical transducer portion of an inkjet recording head in this embodiment has been enlarged.

As shown in this figure, an electromechanical transducer 90 comprises aninterlayer 92, an adhesive layer 81, a bottom electrode 85, aferroelectric thin film (piezoelectric film) 83, and a top electrode 84,all laminated over an oxide film 91 that forms the installation surface.

The interlayer 92, adhesive layer 81, ferroelectric thin film 83, andtop electrode 84 are the same as in Embodiment 5, and therefore will notbe described in detail.

The bottom electrode 85 consists of a first layer 826 and a second layer827. The first layer 826 and second layer 827 are formed as a result ofthe movement of the composition resulting from the manufacturing methodof this embodiment, as discussed below.

The first layer 826 is a layer composed of a specific metal providedover the interlayer 92. This layer is the layered structure resultingafter the iridium (821) provided under the metal layer (822) prior tobaking has been diffused by heat to the metal layer and passedtherethrough. Accordingly, the first layer 826 consists almost entirelyof metal originally present, such as platinum, but crystals of theiridium that has passed through may remain behind.

The second layer 827 is provided over the first layer 826, and is formedas a layer containing iridium. This layer is produced by the separatecrystallization of iridium layers comprising a second iridium layer(823) that has moved in via the metal layer (822).

In the bottom electrode 85 consisting of the first layer 826 and thesecond layer 827, the volumetric ratio of the bottom electrode 85accounted for by the alloy containing iridium (such as an alloy of thetitanium or other metal that has moved from the adhesive layer 81, theoxygen that has escaped from the ferroelectric thin film 83, andiridium) is at least ⅖ and no more than ⅘.

Working Example

An example of manufacturing an electromechanical transducer will now bedescribed through reference to FIG. 19. The baking step in thismanufacturing method is different from that described for FIG. 17. Therest of the steps, such as the oxide film formation step (FIG. 19A, theinterlayer formation step (FIG. 19B), the bottom electrode formationstep (FIG. 19C), the ferroelectric thin film formation step (FIGS. 19Dand E), and the top electrode formation step (FIG. 19F), are the same asin Embodiment 5 given above.

The baking step of this example is characterized in that the baking isperformed at a temperature higher than 750° C. Baking at this relativelyhigh temperature causes the iridium of the first iridium layer 821 todiffuse, and causes this iridium to pass through the metal layer 822 andmove almost completely into the second iridium layer 823.

Also, this heat treatment results in the ferroelectric thin filmprecursors forming a perovskite crystal structure from their amorphousform they had when they were precursors, and changes them into a thinfilm that exhibited an electromechanical transducing action.

After the formation of the various layers of the electromechanicaltransducer, the step of forming the ink jet recording head and thestructure of the printer in which this is utilized are the same as inEmbodiment 5.

With this embodiment, an electrode is sandwiched by an iridium layerover an interlayer, and baked at a temperature higher than 750° C.,which effectively moderates the residual stress in the bottom electrodethrough the diffusion of the iridium, and makes it possible to providean electromechanical transducer having good piezoelectriccharacteristic, and products to which this electromechanical transduceris applied.

The inventors manufactured several electromechanical transducers by theabove method, using zirconium oxide for the interlayer 92, titanium forthe adhesive layer 81, and platinum for the metal layer 822. Thetransducer manufactured by the method of Embodiment 5 with the bakingtemperature set at 700° C. is termed Working Example 1, while thetransducer manufactured by the method of Embodiment 6 with the bakingtemperature set at 850° C. is termed Working Example 2.

FIG. 20 shows the result of measuring the relationship between thepiezoelectric constant d₃₁ and the ratio of the thickness of the firstiridium layer to the thickness of the overall bottom electrode prior tobaking in the electromechanical transducer of Working Example 1. FIG. 21shows the results of measuring the relationship between thepiezoelectric constant d₃₁ and the volumetric ratio of the bottomelectrode accounted for by the alloy containing iridium after baking inthe electromechanical transducer of Working Example 1.

The piezoelectric constant before the pressure chambers were formed andthe residual stress released was 150 pC/N in each case. Forming thepressure chambers tends to moderate the stress and lower thepiezoelectric constant. The lower is this proportional decrease, thebetter are the characteristics.

As can be seen from FIGS. 20 and 22, it was confirmed that regardless ofthe baking temperature, a piezoelectric device that exhibited goodpiezoelectric characteristics was obtained when the thickness ratio ofthe first iridium layer ti the bottom electrode prior to baking was fromabout ⅓ to about ⅘.

As can be seen from FIGS. 21 and 23, it was confirmed that regardless ofthe baking temperature, a piezoelectric device that exhibited goodpiezoelectric characteristics was obtained when the volumetric ratio ofthe bottom electrode accounted for by the alloy containing iridium afterbaking was at least ⅖ and no more than ⅘. The reason this volumetricratio should not exceed ⅘ is that if this ratio is too high, the bottomelectrode will consist almost entirely of iridium and the device will betoo hard and brittle to be used as an actuator for an ink jet recordinghead.

The relationship between the thickness of the adhesive layer, the firstiridium layer, the metal layer, and the second iridium layer prior tobaking, and the thickness of the overall bottom electrode after bakingwas confirm to satisfy the relation of Formula 1 in both workingexamples.

The present invention is not limited to the embodiments given above, andcan be applied with various modifications thereto. For instance, theferroelectric thin film device manufactured not only of the inkdischarge drive source of an manufacture not only of the ink dischargedrive source of an ink jet recording head, but also a nonvolatilesemiconductor memory device, a thin film capacitor, a pyroelectricdetector, a sensor, a surface acoustic wave optical waveguide, anoptical recording device, a space light modulator, a frequency doublerfor a diode laser, and other such ferroelectric devices, dielectricdevices, pyroelectric devices, piezoelectric devices, andelectro-optical devices.

The present invention allows the orientation of a ferroelectric thinfilm to be controlled according to the intended application of aferroelectric thin film device. In particular, the orientation of aferroelectric thin film can be controlled merely by adjusting thethickness of the titanium layer, so there is no need for specialequipment, and orientation can be controlled by a simple method.

The present invention also provides a method for manufacturing anelectromechanical transducer with highly reliable drive characteristic,with which the amount of oxygen contained in a bottom electrode composedof iridium alone can be minimized during the baking of a ferroelectricthin film.

The present invention also comprises an adhesive layer and ananti-diffusion layer, so even if the bottom electrode is made thinnerthan in a conventional product, the adhesive metal will not becomeadmixed in the piezoelectric film and form a low-dielectric constantlayer, and the oxygen and lead in the ferroelectric thin film will notescape from the bottom electrode into the adhesive layer, so there is nodeterioration in piezoelectric characteristics. Also, because the bottomelectrode can be made thinner, there is greater freedom in choosing thematerial and thickness of the diaphragm film an effort to optimize thedisplacement characteristics. Furthermore, reliability is enhancedbecause good adhesion can be maintained even if the ferroelectric thinfilm is made thinner.

Finally, the present invention provides as electromechanical transducer,and products to which this electromechanical transducer is applied, withhigher reliability because residual stress is moderated through themovement of iridium during baking.

What is claimed is:
 1. A method for manufacturing a ferroelectric thinfilm device, comprising the steps of: forming a bottom electrode filmcontaining at least iridium on a surface preparation layer whose maincomponent is zirconium oxide; laminating an ultra-thin titanium layerover said bottom electrode; and forming a crystallized ferroelectricthin film by forming an amorphous layer containing elemental metal andelemental oxygen that constitute a ferroelectric over said titaniumlayer, and heat treating said amorphous layer, wherein the orientationof said ferroelectric thin film is controlled by adjusting the filmthickness in the lamination of said titanium layer.
 2. The method formanufacturing a ferroelectric thin film device defined in claim 1,wherein the priority orientation of said ferroelectric thin film is setto (100) by keeping the film thickness to at least 2 nm and less than 10nm in the lamination of said titanium layer.
 3. The method formanufacturing a ferroelectric thin film device defined in claim 1,wherein the priority orientation of said ferroelectric thin film is setto (111) by keeping the film thickness to at least 10 nm and less than20 nm in the lamination of said titanium layer.
 4. The method formanufacturing a ferroelectric thin film device defined in any claims 1to 3, wherein the step of forming said ferroelectric thin film is a stepof forming a film by sol-gel method from a ferroelectric whoseconstituent components are at least titanium and lead.
 5. The method formanufacturing a ferroelectric thin film device defined in claim 4,wherein said ferroelectric is lead titanate zirconate.
 6. The method formanufacturing a ferroelectric thin film device defined in any of claims1 to 3, wherein the step of forming said lower electrode is a step offorming a single layer of an iridium film, or using platinum and iridiumto form a laminate film comprising an (iridium layer)/(platinum layer),a (platinum layer)/(iridium layer), or an (iridium layer)/(platinumlayer)/(iridium layer), in that order starting at said surfacepreparation layer.
 7. The method for manufacturing an ink jet recordinghead, comprising the steps of: forming a surface preparation layer whosemain component is zirconium oxide on a silicon substrate surface, eitherdirectly or via a diaphragm film; forming a bottom electrode containingat least iridium over said surface preparation layer; laminating atitanium layer whose film thickness is at least 10 nm and less than 20nm over said bottom electrode; forming a ferroelectric thin film havinga priority orientation of (111) by forming an amorphous film containingthe elemental metal and elemental oxygen that constitute theferroelectric over said titanium layer and then treating said amorphousfilm; manufacturing an electromechanical conversion device by forming antop electrode over said ferroelectric thin film; and separating theelectromechanical transducer so as to line up with the position wherethe mechanical displacement of the electromechanical transducer can beimparted to the pressure chamber.
 8. The method for manufacturing an inkjet recording head, comprising the steps of: forming a surfacepreparation layer whose main component is zirconium oxide on a siliconsubstrate surface, either directly or via a diaphragm film; forming abottom electrode containing at least iridium over said surfacepreparation layer; laminating a titanium layer with a thickness of atleast 2 nm and less than 10 nm over said bottom electrode; forming aferroelectric thin film having a priority orientation of (100) byforming an amorphous layer containing elemental metal and elementaloxygen that constitute a ferroelectric over said titanium layer, andheat treating said amorphous layer; manufacturing an electromechanicaltransducer by forming an top electrode over said ferroelectric thinfilm; and separating the electromechanical transducer so as to line upwith the position where the mechanical displacement of theelectromechanical transducer can be imparted to the pressure chamber. 9.The method for manufacturing an ink jet recording head, according toclaim 7 or 8, wherein the step of forming said ferroelectric thin filmis a step of forming a film by sol-gel method from a ferroelectric whoseconstituent components are at least titanium and lead.
 10. The methodfor manufacturing a ferroelectric thin film device defined in claim 9,wherein said ferroelectric is lead titanate zirconate.
 11. The methodfor manufacturing an ink jet recording head defined in any claims 7 or8, wherein the step of forming said bottom electrode is a step offorming a single layer of an iridium film, or using platinum and iridiumto form a laminate film comprising an (iridium layer)/(platinum layer),a (platinum layer)/(iridium layer), or an (iridium layer)/(platinumlayer)/(iridium layer), in that order starting at said surfacepreparation layer.
 12. A method for manufacturing a nonvolatileferroelectric memory device, comprising the steps of: forming a bottomelectrode containing at least iridium over a surface preparation layerwhose main component is zirconium oxide; laminating a titanium layerwhose film thickness is at least 2 nm and less than 10 nm over saidbottom electrode; and forming a capacitor insulating film having apriority orientation of (100) by forming an amorphous film containingthe elemental metal and elemental oxygen that constitute theferroelectric over said titanium layer and then heat treating saidamorphous film.
 13. The method for manufacturing a nonvolatileferroelectric memory device according to claim 12, wherein the step offorming said capacitor insulating film is a step of forming a film bysol-gel method from a ferroelectric whose constituent components are atleast titanium and lead.
 14. The method for manufacturing a nonvolatileferroelectric memory device defined in claim 13, wherein saidferroelectric is lead titanate zirconate.
 15. The method formanufacturing a nonvolatile ferroelectric memory device defined in anyclaims 12 to 14, wherein the step of forming said bottom electrode is astep of forming a single layer of an iridium film, or using platinum andiridium to form a laminate film comprising an (iridium layer)/(platinumlayer), a (platinum layer)/(iridium layer), or an (iridiumlayer)/(platinum layer)/(iridium layer), in that order starting at saidsurface preparation layer.
 16. A method for manufacturing anelectromechanical transducer, comprising the steps of: forming a bottomelectrode containing at least iridium over a surface preparation layerwhose main component is zirconium oxide; laminating a titanium layerwith a thickness of at least 2 nm and less than 10 nm over said bottomelectrode; and forming a ferroelectric thin film having a priorityorientation of (100) by forming an amorphous layer containing elementalmetal and elemental oxygen that constitute a ferroelectric over saidtitanium layer, and heat treating said amorphous layer.
 17. The methodfor manufacturing an electromechanical transducer to claim 16, whereinthe step of forming said ferroelectric thin film is a step of forming afilm by sol-gel method from a ferroelectric whose constituent componentsare at least titanium and lead.
 18. The method for manufacturing anelectromechanical transducer defined in any claim 17, wherein saidferroelectric is lead titanate zirconate.
 19. The method formanufacturing an electromechanical transducer defined in any of claims16 to 18, wherein the step of forming said bottom electrode is a step offorming a single layer of an iridium film, or using platinum and iridiumto form a laminate film comprising an (iridium layer)/(platinum layer),a (platinum layer)/(iridium layer), or an (iridium layer)/(platinumlayer)/(iridium layer), in that order starting at said surfacepreparation layer.
 20. A method for manufacturing an electromechanicaltransducer, comprising the steps of: forming a bottom electrode composedof iridium alone over a surface preparation layer whose main componentis zirconium oxide; laminating a titanium layer whose film thickness isat least 15 nm and less than 30 nm over said bottom electrode; andforming a crystallized ferroelectric thin film by forming an amorphousfilm containing the elemental metal and elemental oxygen that constitutethe ferroelectric over said titanium layer and then heat treating saidamorphous film.
 21. A method for manufacturing an electromechanicaltransducer, comprising the steps of: forming a bottom electrode composedof iridium alone over a surface preparation layer whose main componentis zirconium oxide; laminating an ultra-thin titanium layer over saidbottom electrode; and forming a crystallized ferroelectric thin film byforming an amorphous film containing the elemental metal and elementaloxygen that constitute the ferroelectric over said titanium layer andthen heat treating said amorphous film, wherein the priority orientationof the ferroelectric thin film is controlled to the (111) plane or the(110) plane by adjusting the thickness of said titanium layer to atleast 15 nm and less than 30 nm.
 22. The method for manufacturing anelectromechanical transducer according to claim 20 or 21, wherein thestep of forming said ferroelectric thin film is a step of forming a filmby sol-gel method or MOD method.